The present invention relates to a method of manufacturing a zirconium-based alloy tube for a fuel element sheath to be used in a nuclear reactor, comprising a plurality of successive rolling and annealing steps.
The invention is particularly suitable, although not exclusively, in the field of manufacturing nuclear fuel element sheaths for pressurized water reactors, called PWRs, which sheaths are formed from a zircomium-based alloy.
The most widely used alloys include particularly the so-called "Zircaloy" alloys, among which are "Zircaloy-2" and Zircaloy-4", having contents to table I.
TABLE I ______________________________________ Element % in weight ______________________________________ Zircalov 2 Sn 1.2-1.7 Fe 0.07-0.20 Cr 0.05-0.15 Nr 0.03-0.08 Zr + complement impurities Zircalov 4 Sn 1.2-1.7 Fe 0.18-0.24 Cr 0.07-0.13 Zr + complement impurities ______________________________________
Zircaloy alloys have a low neutron absorption, and have generally acceptable mechanical strength and satisfactory stress-corrosion resistance.
It is nevertheless important to improve these qualities in order to keep the fuel elements in the reactor during a longer time (increasing their burn-up) and to further reduce the risks of creating cracks within the fuel sheaths. For this purpose, different heat treatments have been recommended during manufacture of the tubes to be used in a reactor.
In a manner known per se, the zircaloy tubes used for the fuel element sheaths, but also for the probe guide tubes for the tubes of clusters of control elements, controlling the nuclear reactors, are manufactured from hot worked ingots.
A first step consists in forging the ingot at a first range temperature called ".alpha. range" before heating the ingot to a second range temperature called ".beta. range" and then rapidly cooling the ingot with water to ambient temperature. The billet is then extruded in .alpha. range, before cold rolling operations. Several successive rolling and annealing passes in .alpha. range are then carried out in a conventional manner.
By ".alpha. range" should be understood the temperature range in which the crystalline structure of the alloy is hexagonal and close packed (.alpha. phase), and by ".beta. range" the temperature range in which the crystalline structure of the alloy is body centered cubic (.beta. phase).
The transition temperature from .alpha. phase to .beta. phase of pure zirconium is 862.degree. C.
Zirconium-based alloys have an intermediate range, where the two crystalling structures .alpha. and .beta. are present. This range, where the alloy is in the so-called .alpha.+.beta. phase, extends in the temperature range between 825.degree. C. and 950.degree. C.
Attempts have been made in different ways to improve the above-described manufacturing method, for example, by attempting to improve quenching conditions from the .beta. phase (FR-2 244 831) or by providing, before the last rolling pass, a passage in .beta. phase (U.S. Pat. No. 3 865 635) with better resistance as a claimed result.
In most cases, a final heat treatment of the tube is performed, consisting of a stress-relief and recrystallization in .alpha. phase annealing. To date, this final heat treatment has been considered necessary by those skilled in the art for obtaining an alloy having both generalized stress-corrosion resistance and acceptable mechanical strength under irradiation.
Methods for manufacturing tubes or structural parts for nuclear reactors, involving a final heat treatment in the .beta. range, also exist for improving "pustular" corrosion resistance of the structural parts to reduce spalling of oxide flakes appearing on surfaces of said structural parts under normal operating conditions, particualarly under boiling water reactor conditions.
Thus, Pat Nos. US-4238251, DE-2951102 and DE-2951096 teach the use of different heating methods (induction, laser), and a final .beta. range heat treatment of the structural parts. This .beta. range treatment is however only a surface treatment and is only applied to structural parts which will not be in direct contact with nuclear fuel and therefore not subjected to stress-corrosion, contrary to tubes used for fuel cladding.
On the contrary, it should be noted that, when these patents mention applications of the methods to manufacture sheaths of fuel elements, they no longer teach .beta.-phase treatment of the tubes, thought to be deterimental to corrosion resistance and mechanical strength in that case, but rather an .alpha.+.beta. transition phase treatment (between 860.degree.-930 .degree. C.), as it is known in equivalent prior art.